1. Field of the Invention
This invention relates to the field of differential amplifiers, especially operational amplifiers, and more particularly, to the correction of the offset voltage of these amplifiers. It also relates to a macro-functional circuit which operates as a digital analog converter.
In certain cases the information or input data of a circuit is given by the resistor value. This applies especially to the read circuits of magnetic bubble memories. The presence or absence of a bubble at a given point is indicated by a change in the value of the two resistors R1 and R2. If a bubble is present, the resistor pair (R1, R2) is given a first value (R1S, R2S); if no bubble is present, the pair is given a second value. The detection of the bubble presence or absence status is performed simply by causing a current to flow through each of the resistors R1 and R2 and by measuring the difference of the voltage drop V.sub.E =R1S.I1-R2S.I2 in a differential amplifier (see FIG. 1). However, in order to reduce static and dynamic errors due to the asymmetry of the elements of the input circuit (i.e. R1, R2 and I1, I2), the latter must be such that the two inputs of the differential amplifier have the same impedance; this means that R1=R2=R and I1=I2=I.
If the input signal V.sub.E is absent, the offset voltage at input .DELTA.V.sub.O (which will be explained later) is such that .DELTA.V.sub.O =R10I1-R20I2, R10 and R20 being the values of the resistors R1 and R2 and when no signal is present.
The advantage of this circuit is that it is now completely symmetrical, which allows for compensation of the influence of the input current and the resistor temperature drifts. It also allows for compensation of the noise effects generated by the rapid voltage variations and the parasitic capacities (V.sub.NOISE =RC dV/dt).
The major problem related to this type of circuit lies in the significant offset voltage which appears between the two amplifier inputs. This voltage is produced by the static difference between the R1 and R2 values on the one hand, and the I1 and I2 values on the other hand, whereas it is desirable that these values be equal. For certain approximations, the relations R1=R2=R and I1=I2=I are acceptable. This offset voltage is important enough to justify compensation.
2. Background Art
It will be understood that a number of solutions have been attempted in the art for the problem described above. For example, correction by means of a potentiometer is possible under certain circumstances, but this is a costly and rather unreliable solution.
Correction by the so-called active trimming technique is not possible in cases where R1 and R2 are outside the circuits. A dynamic compensation method is therefore required for this offset voltage. The compensation method known as chopper stabilization allows for offset compensation. For example, reference could be made to the standard manual Operational Amplifier Design and Applications, by Burr-Brown, McGraw-Hill, pages 150 to 164. The principle of this technique is to separate the DC component by means of an LF amplifier. An equivalent representation for a differential amplifier is illustrated in FIG. 2. The LF amplifier must have a cutoff frequency of the order of several KHz, which requires the use of relatively high capacitances (30 to 50 picofarads). However, the values of the capacitors which can be implemented in monolithic circuits are rather limited because they need too much of the surface and also considerably reduce the production capacity. Another disadvantage is the increase in the number of capacitors for each amplifier input and the nonsymmetry of these inputs.
FIG. 3 illustrates a further possible compensation method. The gain of the input amplifier is low enough to allow it to operate in the linear region regardless of the offset voltage at the input; that is, the product of the gain multiplied by the offset voltage is lower than the maximum voltage admissible at the output of this amplifier. The DC component is then suppressed by capacitor C and the AC component is amplified by the second amplifier. The disadvantage of this approach is the low input stage gain, in addition to the capacitor problem already mentioned. In effect, the rejection rate in common mode is proportional to the amplifier gain, and when the input offset voltage is high, the gain must be low (2 to 5).